The present invention relates to a deployment and retrieval system for a marine seismic source and more particularly relates to a system to deploy and retrieve a seismic source assembly from the stern of a marine seismic vessel in a safe and efficient manner.
In a typical marine seismic operation, an electronically equipped vessel tows both a seismic source assembly and a seismic cable or streamer through an area to be investigated. The source assembly is actuated to generate signals which in turn reflect off various strata underlying the marine bottom. These reflected signals are received by each of a plurality of receivers, e.g. hydrophones or the like, which are spaced along the length of the streamer. The received signals are recorded and processed to produce the desired seismic record.
In recent years, seismic source assemblies have become more and more sophisticated and, hence, more difficult to deploy and retrieve. Today, it is common to have a plurality of sources (e.g. chambers adapted to instantaneously discharge a burst of compressed air under high pressure) spaced along a flexible core or hose which, in turn, may extend several hundred feet behind a vessel when fully deployed. Conduits for supplying air to the chambers and lines for actuating the chambers and for transmitting data from the chambers to the vessel are all encased in the assembly. Also, a strain absorbing member, e.g. wire rope, is normally encased in the hose and runs throughout its length to protect the hose assembly from possible damage due to the strain experienced as the assembly is towed behind the vessel.
As an added precaution against strain damage, it is common to deploy a relatively heavy strain absorbing element, e.g. a chain, along with the hose assembly and attach the hose assembly to the chain at spaced points as both are deployed from the vessel. The major portion of the strain experienced by the hose assembly during towing will be absorbed by the chain. Also as the chain and the hose assembly are deployed, buoys are attached thereto usually at the same points as where the sources are attached. The buoys not only mark the path of the seismic hose assembly but also aid in maintaining it at a desired depth during operation.
At the end of a seismic operation, the seismic hose assembly is retrieved onto the vessel. The buoys are detached from the hose assembly and the chain is also detached and coiled onto a reel for storage. The sources, themselves, normally remain attached to the hose assembly and are stacked and stored as a unit on columns or bulkheads on the vessel, as is known in the art.
Previously, the deployment and retrieval of the seismic hose assembly has involved a delicate and difficult procedure. The chain was merely positioned within an open guide trough on the deck of the ship with each seismic source being attached to the chain during deployment while both source and the chain are lying side by side on the deck. A source after it was attached was then dragged across the deck (approximately 25 feet) and a buoy line was attached just before the source was deployed over the stern. Not only did this present possible damage to the expensive sources and related electrical and pneumatic llines but more importantly, also presented a less than desirable condition for the personnel handling the source assembly. For example, heaving of the vessel due to rough weather or the like could cause the chain to jump out of the open track and whip wildly about the deck, endangering personnel nearby. When the source assembly was retrieved, the chain and the attached sources were pulled roughly over the stern onto the deck and both the source and its control lines could easily be damaged during this operation. Accordingly, the need for a safer and more efficient deployment and retrieval system is evident.